Fix RegularGrid issues with data_vars and decreasing coordinates

[Nesar976]

This PR fixes multiple issues in RegularGrid that were causing errors and incorrect subsetting.

• Fixes the data_vars logic, which was broken and raising errors due to incorrect access on variable names.
• Adds proper support for grids where latitude or longitude coordinates are decreasing instead of increasing.
• Removes assumptions about static slice direction and computes slices dynamically based on coordinate order.

The fixes were verified using a reproduction script and existing tests to ensure no regressions.

Fixes #97